AAT Images of Fragment G Impact

AAT IRIS Observations of the Impacts of
Fragment G of Shoemaker-Levy 9 with Jupiter

The InfraRed Imaging Spectrometer (IRIS) is being used on the 3.9 m
Anglo-Australian Telescope (AAT) at Siding Spring Observatory (near
Coonabarabran, Australia) to monitor the impacts of the fragments of Comet
Shoemaker-Levy 9 with Jupiter. On 18 July, observations of
Jupiter were initiated at 6:30 UT, about one hour before sunset. The skies
were clear, and the seeing was better than 1 arc-second. Near-infrared images
and spectra at wavelengths between 2 and 2.4 microns taken between 6:30 and
7:30 UT on 18 July 1994 revealed two bright features near the evening
terminator at about 45 South latitude. These features were identified as the
impact sites of fragments A and C, which had been monitored on the previous
day. Bright flashes associated with the impacts of fragments G2 and G1 were
first detected on the morning limb of Jupiter at 7:33 and 7:40 UT,
respectively. The impact flashes were tens to hundreds of times brighter than
those seen previously. These sites were monitored until 13:30 UT.

Instrumentation and Observing Technique:

IRIS is a near-infrared camera/spectrometer with a 128 by 128 element
Mercury-Cadmium-Telluride (NICMOS2) detector. This instrument can be used for
direct imaging at wavelengths between 0.9 and 2.5 microns, or for the
acquisition of spatially-resolved near-infrared spectra of extended objects.
To monitor the G fragment impact events, IRIS was used primarily as an imaging
spectrometer. Images of the Jovian disk were obtained simultaneously at 128
wavelengths (or colors) within H-band (1.4 to 1.8 micron) and K-band (2.0 to
2.4 microns). These "spectral image cubes" were collected using the IRIS H
and K grisms which provide a spectral resolution of 300, and a spatial
resolution of 0.6 arc-seconds per picture-element along a 1 by 60 arc-second
slit. Full-disk images of Jupiter were compiled by allowing the planet to
drift perpendicular to the slit while recording spectra (which are dispersed
perpendicular to the slit) at slit positions offset by 0.6 arcsec. With a 1.5
second sampling time at each slit position, a complete image of the Jovian
disk can be produced in about two minutes.

The Evolution of the Fragment G Impact Sites:

IRIS K-grism cubes revealed a faint flash on the morning limb of
Jupiter at 7:33 UT. This feature was initially much dimmer than the C-impact
site, which was still on the evening limb of Jupiter. The brightness of this
feature increased by about a factor of 10 by 7:35 UT and remained stable until
about 7:40 UT. At that time, the morning limb suddenly brightened
dramatically, saturating the detector, producing a brilliant star-like image
with diffraction spikes. To bring the detector back on scale, the 3.9 meter
AAT mirror was partially closed, reducing the effective aperture of the
telescope to less than 1 meter (less than one tenth of its full size).

After 8:10 UT, the G impact site had rotated onto the Earth-facing
hemisphere of Jupiter, and its surface brightness at wavelengths near
2.34-microns decreased to about 12 times that of the south polar hood. The
size of the feature was comparable to that of the Great Red Spot. Unlike the
impact sites of the earlier fragments, the G impact site was visible at all
wavelengths where observations were collected. It was even visible in the TV
camera that was being used to guide the telescope. This TV camera employed a
spectral filter centered at 0.55 microns, and revealed the impact site as
low-contrast dark spot with dimensions comparable to those seen at 2.34
microns. A similar feature was seen in IRIS images taken at 1.083 microns.
IRIS H-grism images taken within the 1.7 micron methane band showed a 15000 km
diameter bright feature surrounded by a dark apron, which had about twice that
diameter. IRIS K-grism images taken within the strong methane bands near 2.34
microns resolved the site into a double-lobed feature. Hubble Space Telescope
observations suggest that the second smaller component of this feature may
have been associated with the impact site of fragment D, which was monitored
on the previous evening (Heidi Hammell, personal communication, 1994). Peter
McGregor and Mark Allen used the CASPIR infrared camera at the 2.3 m
Australian National University Telescope at Siding Spring Observatory to
acquire observations at wavelengths between 3 and 4 microns. At these
wavelengths, they detected a bright spot 2000 to 3000 km in diameter,
surrounded by distinct 30,000 km diameter bright ring.

The impact of the first of the G fragments was detected at about 45 S latitude
on the morning (left-hand) limb. This impact event is most visible at the
longer wavelengths, where the methane bands are strongest. Images 3 and 4
were taken at 7:35 and 7:39 UT, as the peak brightness of this feature
increased to more than 10 times the surface brightness of the C impact site
and the south polar cap. The 5th image shows the initial stages of the rapid
brightening which started just after 7:41 UT. The peak brightness of this
feature increased to roughly 400 times that of the south polar hood at about
7:47 UT. The last image shows this feature at 7:58 UT, after its surface
brightness decreased to about 12 times that of the south polar hood.

The long-term evolution of the G-impact site is shown in this image.
The three columns of images in the
image show the appearance of the Jovian disk at 8:45, 9:30, and 12:05
UT.
the Jovian disk at 7:55, 9:30, and 12:05 UT.

In H-band, the evolution of the G impact site is most obvious in the
bottom row of three images, which were created by summing H-band images at
wavelengths between 1.67 and 1.75 microns, within a strong methane band. The
images in the top two panels were extracted at wavelengths spanning 1.48 to
1.61 microns, and 1.56 to 1.61 microns, where the contrast of the impact site
is much lower.